1. Field of the Invention
The present invention relates to a solenoid valve for opening and closing a passage to allow or stop a flow of fuel gas to a domestic gas device such as gas water heater.
2. Description of the Related Art
The amount of an electric current applied to a solenoid of a solenoid valve when the solenoid valve is kept open is smaller than the amount applied when the solenoid valve is to be open when closed. Nevertheless, even when the solenoid valve is kept open, if magnetic leak occurs from a gap between an armature and a core energized when an electric current is applied to the solenoid, the amount of the electric current needed to keep the solenoid valve open is larger than desirable.
In Japanese Unexamined Utility Model Publication No. 64-17075, one of the present inventors proposed a solenoid valve having an additional solenoid, i.e., in addition to a usual solenoid, for producing a magnetic flux in a fully closed magnetic path so that only a small electric current is needed to keep the solenoid valve open.
FIGS. 1 and 2 show a construction of the proposed solenoid valve. In the drawings, a core 10 made of a usual magnetic soft-alloy has a column portion 11, a flange portion 12 formed at one end of the column portion 11, and a support portion 13 formed at the other end of the column portion 11. A first solenoid 14 is provided adjacent to the column portion 11, and a second solenoid 15 is located in an annular groove 16 formed on an outer end face of the support portion 13. A guide rod 17 is connected to the center of a bottom wall 21 of a cup-shaped plunger or armature 20 by a solder 19, and inserted in a guide hole 18 formed in the axial portion of the core 10 to be slidably supported therein.
A valve body 25 is provided at the end portion of the guide rod 17, and is urged by a spring 26 in a direction in which the valve body 25 closes a passage (not shown). As shown in FIG. 1, an electric current is applied to the solenoids 14 and 15 so that the plunger 20 is attracted by the core 10, and thus the valve body 25 is opened against the force of a spring 26. Namely, an inner face of the bottom wall 21 comes into contact with an outer end face of the support portion 13, and thus an annular space 22 is formed between a lower end peripheral portion 23 and the flange portion 12.
The solenoid valve is operated as follows. When the valve body 25 is positioned upward in the drawings, i.e., has closed the passage, an inner face of the bottom wall 21 is separated from an end face of the support portion 13, and the lower end peripheral portion 23 of the plunger 20 and the flange portion 12 are separated from each other by a wide gap. Therefore, to open the valve body 25, a large electric current must be applied to the first solenoid 14, so that the plunger 20 is attracted to the core 10, and after the valve body 25 is opened, a small electric current applied to the second solenoid 15 to keep the valve body 25 open, and the application of electric current to the first solenoid 14 stopped. Namely, the valve body 25 is kept open by the small electric current. An electric current may be applied to the first and second solenoids 14 and 15 at the same time, to open the valve, and after the valve body 25 is opened, the application of electric current to the first solenoid 14 may be stopped. Accordingly, since magnetic path generated by applying an electric current to the second solenoid 15 is formed as a fully closed magnetic circuit, through the bottom wall 21 and the support portion 13, the amount of electric current to be applied to the second solenoid 15 to keep the valve body 25 open is reduced.
The solenoid valve having the above construction, however, has the following problems.
If the guide rod 17 is fixed to the plunger 20 by the solder 19 in such a manner that the guide rod is inclined to the axis of the plunger 20, as shown in FIG. 2, the entire surface of the inner face of the bottom wall 21 does not come into contact with the end face of the support portion 13, and as a result, after the valve is open, a large amount of the electric current must be applied to the second solenoid 15 to keep the valve open. Similarly, if an attracting force generated at the lower end peripheral portion 23 to attract the peripheral portion 23 to the center of the core 10 is not balanced along the entire periphery of the portion 23, the solder 19 may be elastically deformed and the plunger 20 hereby inclined against the guide rod 17, so that the entire furface of the inner face of the bottom wall 21 does not come into contact with the end face of the support portion 13.
Further, if an attracting force generated at the lower end peripheral portion 23 to attract the portion 23 to the center of the core 10 is not balanced along the entire periphery of the portion 23, the guide rod 17 may be inclined toward the guide hole 18, whereby the lower end of the rod 17 comes into contact with the inner wall of the guide hole 18, and as a result, the guide rod 17 is not smoothly guided by the guide hole 18 and a smooth movement of the plunger 20 is not obtained. Also, if the plunger 20 is inclined due to the unbalanced force, so that a part of the portion 23 is in contact with the flange portion 12, the downward movement of the plunger 20 is obstructed.
Both the magnetic flux 28 generated by applying an electric current to the first solenoid 14 and the magnetic flux 29 generated by applying an electric current to the second solenoid 15 pass through the bottom wall 21, and when the valve is opened from a closed state, a high electric current having a high magnetization intensity is applied to the first solenoid 14. Therefore, due to a remanence existing on the bottom wall 21 after shutting off the electric current, the plunger 20 is attracted to the core 10 even after the electric current applied to the second solenoid 15 is shut off, whereby the opening movement of the valve is delayed.
This delay is also caused by a remanence of the bottom wall 21 generated by applying an electric current to the second solenoid 15.